(1). Field of the Invention
The present invention relates to a method of removing liquids from water containing resin of high melting point over 200.degree. C., and more particularly to a monitoring system for removing liquids from the high melting point resin by melt-kneading in a co-rotating twin screw extruder for removing the water component with extrusion, and method and apparatus as well as molding thereof.
(2). Background Information
Generally, resin materials produced in a polymer manufacturing process of thermoplastic resins or ground materials produced in a recycling process of the waste plastic products (hereinafter simply referred to as resin materials) contain a relatively large quantity of liquid so that a liquid removing process is required to remove the water component at the end of each process.
Hitherto, in a manufacturing process of ABS (acrylonitrile butadiene styrene) resins for example, a flow drying oven and the like have conveniently been used. ABS latex manufactured by an emulsion polymerization may be obtained as a slurry which contains a large quantity of liquid after coagulation and water washing. As a method of removing liquids, the slurry is supplied into a centrifugal dehydroextruder for obtaining a water containing powder or a wet cake containing water in the range of 30%-40%. The water containing powder is dried until the water content reaches 1% by heat energy using a drying pipe and a flow drying oven to avoid compounding by an extruder in the final process, a vent-up and a blowing phenomenon in a granulation process.
In the drying process, the approximately 30%-4% water component of the powder or cake is vaporized by consuming large quantities of heat energy and time. The powder of lower water content in almost dried state is associated with a risk of dust explosion so that the device as a whole needs to be isolated from oxygen with a hot nitrogen gas and a larger scale of the facility.
A resin material of approximately 1% water content is supplied to a co-rotating twin screw extruder for the ultimate dryness followed by compounding and granulating.
For a scale down version of the device, a method of removing liquids by means of a co-rotating twin screw extruder has been proposed. A water removing system of this type generally comprises a squeeze process and a vaporization process.
In the liquid removing method of the squeeze type as illustrated in FIG. 9, for example, a co-rotating twin screw extruder 10 is mounted with a co-rotating twin screw 14 in a barrel 12 and provided with a material feeding section (or port) 16 to which is supplied a mixture of a water containing powder 30a, a pellet 30b and a process adjuvant 30c of the resin material 30 from which liquids are previously removed by a centrifugal hydroextruder 32. It is proposed to provide such structure so that the resin material within the mixture is squeezed at the liquid removing screw portion (not shown) of the screw 14 so that the resin material is mold-extruded as a solid from a delivery (outlet) 18.
ABS water containing powder 30a of water content of 30%-40% reduced by the centrifugal hydroextruder 32 is supplied to the material feeding section 16 at the uppermost stream of the co-rotating twin screw extruder 10 together with other materials such as SAN (styrene and acrylonitrile copolymer) pellet 30b and the process adjuvant 30c. The material supplied to the material feeding section 16 is conveyed effectively to the lower stream without retention by the twin screw 14 having a self-cleaning finction. The material being conveyed is squeezed by a liquid removing screw portion 24 comprising a kneading block and a reverse-flighted screw provided in the twin screw 14, whereby the material is elastically or plastically deformed for reduction of the specific volume of the material so that the water component in the material is removed as a liquid.
The liquid separated flows in reverse through the screw channel against the upper stream of the liquid removing screw portion 24 along a pressure gradient directing from a maximum pressure point of the liquid removing screw portion to the material feeding section 16 and is discharged from a delivery 20 (water removing screen) provided between the upper stream of the liquid removing screw 24 and the material feeding section 16. At this section, approximately 30%-90% of the entire water component in the resin material is separated and discharged, and approximately 10%-70% of the remaining water component is separated and vaporized through vent holes 22a, 22b and 22c of the vent portion provided at the lower stream of the liquid removing screw, and is finally vaporized through the vent hole 22c and the resin is extruded from the delivery 18.
According to the liquid removing method of the aforementioned type, a certain quantity of water is removed from the resin material in the form of liquid without vaporization so that water vaporization energy effectiveness may be improved. Since the wet material is used, a risk of dust explosion may be avoided without any particular device for sealing oxygen with minimized scale of a facility. Further, in accordance with the liquid removing method of this type, continuation of the manufacturing process, simplification of the multi-stage processes and drastic rationalization of the manufacturing process may be achieved.
In the liquid removing method of the squeeze process as proposed in FIG. 10, for example, a method of removing the liquid is similar to the aforementioned system but a method of discharging the liquid is different. Namely, in the former method of the squeeze process, a separated liquid is flowed in reverse and discharged from the liquid removing screen located in the upper stream of the liquid removing screw portion. In the process shown in FIG. 10, however, to separate and evaporate water the liquid removing portion 34 is provided with a discharge port 35 at a portion where the resin material is compressed and with a vent staffer means 36 to prevent a material from flowing out the discharge port. Other structures are the same as those of the liquid removing method as hereinbefore described.
The liquid removing method free of water reverse flow may expand the operational conditions and this method is well tolerated to a recycling process of the waste plastics.
Further, another liquid removing method of the vaporization process has been proposed as illustrated in FIG. 11 where SAN pellet and water containing ABS powder are supplied into the twin extruder 10 from different places.
Namely, to remove the liquid SAN (styrene and acrylonitrile copolymer), a pellet 30b is supplied to the extruder 10 for quick melting, to which preheated water containing ABS powder 30a with the process adjuvant are added by means of a side feeder 37 having a preheating means in the middle portion of the extruder. Thus, the water component is steamed by an enthalpy of the molten SAN so that the separated and vaporized steam is discharged from the vent holes 22a, 22b and 22c of the vent portion 22.
Accordingly, in the liquid removing method of the type as hereinbefore described, the wet material is also used to avoid any risk of the dust explosion without necessitating any facility to isolate oxygen and to minimize an arrangement. A residual monomer value obtained after devolatilization from the vent hole 22c is better than that of the liquid removing method of the squeeze process as hereinbefore described and the liquid removing method of this type is well tolerated in the case where a material of two components, for example, containing ABS diluted by SAN is adopted.
In the aforementioned conventional liquid removing method of the squeeze process, however, a melting point, a glass transition point or a heat distortion temperature is extremely high as compared with a boiling point of water, and when liquid is removed from the solid resin material of more than 10% water content by means of the co-rotating twin extruder, more power is required as compared with that of the material of low melting point such as ABS. Namely, according to the conventional liquid removing method, a water containing material is squeezed by a propelling force of the screw at the squeeze liquid removing screw comprising a kneading block and/or a reverse-flighted screw provided in the extruder in order to remove liquid by an elastic or plastic deformation. The materials of low melting point such as ABS have the melting points which are similar to or lower than the boiling point of water so that they are elastically or plastically deformed at a relatively lower temperature and pressure by a minimized energy of the screw for the material and a smaller power of the extruder.
Materials of high melting points such as engineering plastics, however, have a melting point, a glass transition point or a heat distortion temperature that is extremely higher than the boiling point of water and may not conveniently be elastically or plastically deformed. For convenient deformation, a temperature of the material at the squeeze liquid removing screw portion may be elevated, notwithstanding that a heat energy may not be supplied to satisfy the necessary evaporation latent heat. Accordingly, it is inevitable to increase the pressure of the squeeze liquid removing screw portion to an extremely high level by applying a large quantity of energy from the screw to the material resulting in an expansion of the power supply. In the co-rotating twin screw extruder where the screw torque is limited on account of the mechanical strength, a large power is required to remove the liquid with reduction of the processing capacity and deterioration of the energy effectiveness losing advantages described hereinbefore.
For example, when resins of high melting points are subjected to the liquid removing procedure without using any particular screw for increasing the pressure at the squeeze liquid removing screw portion or any other means necessary for increasing the power, water is not discharged from the discharge port and a flake up phenomenon occurs to flow the material up from the vent portion at the lower stream. To remove the liquid from the resins of high melting points using such a conventional liquid removing method results in loss of the advantages of liquid removal molding by means of the twin screw extruder which purports to provide an improvement in the energy effectiveness with worthlessness as an industrial facility.
The aforementioned conventional liquid removing method of the vaporization process is effective to the materials containing more than two components of a combination of the water containing materials with other resins but is not applicable to the engineering plastics containing of essentially one component and having a high melting point.
The liquid removing method of this kind stands on an energy balance between water containing materials and other materials. Generally, an evaporation latent heat is greater than an enthalpy of molten resin, and the liquid remove molding method may not be carried out because of loss of the energy balance when water content of the resin material is increased on account of increments of the water content and mixing ratio of the water containing materials, and in this situation, a flake-up phenomenon appears to fly up the material from the vent portion at the lower stream. According to the heat balance of the ABS liquid remove molding method, the water content of the resin material is generally limited to the range of 8%-10%.
The conventional liquid removing method is not applicable to the liquid remove molding of the engineering plastics of essentially one component and even if applicable, the scope of the application is extremely limited because of the energy balance.